Abell 520 is one of the most gigantic mergers of galaxies we've ever seen, and it's definitely the most baffling. As galaxies smashed together and parted away, they seemingly left their dark matter behind. That's supposed to be completely impossible.

It's so impossible, in fact, that when astronomers first detected the "abandoned" dark matter in Abell 520 back in 2007, they figured the data had to be wrong. Considering Abell 520 is 2.4 billion light-years away, a bit of observation error was hardly out of the question. But now, five years later, the Hubble telescope has double-checked and confirmed the original findings. This doesn't fit with our established theories of dark matter and galaxy formation.

So what exactly is going on in Abell 520? Basically, the whole thing is a merger of different galaxy clusters that has been going on for eons. At the center of the emerging cluster are a bunch of galaxies that have been thrown together. All around them is a "dark core" of, naturally enough, dark matter. But the amount of matter in that dark core is far more than can be traced to the galaxies in the core, which must mean other galaxies left their dark matter behind as they traveled through this chaotic region of space.

That really shouldn't be possible, since dark matter is supposed to be what holds galaxies together. We first deduced its existence about eighty years ago when astronomers Jan Oort and Fritz Zwicky realized there was a major discrepancy between the mass of galaxies based on the amount of matter we could see and their observed gravitational effects. Dark matter is meant to account for that gap and serve to keep galaxies from flying apart - so the fact that galaxies seemingly ejected lots of their dark matter in the Abell 520 merger just doesn't make any sense.

Still, even though we still aren't necessarily sure of its precise nature, we have eighty years of strong experimental and theoretical evidence backing up its existence. As such, this isn't a moment where we should throw up our hands and shout, "Ha! Scientists are wrong about dark matter!" This is one piece of evidence that seemingly contradicts a whole lot of other data backing up the current dark matter model - check out Dr. Dave Goldberg's primer if you want the specifics - but it does seem to be one of the most compelling challenges to the current model that we have yet encountered.

On today's episode of our "Ask A Physicist" series, Dr. Goldberg tackles the darker…
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In a statement released by the Hubble site, physicists readily admitted that this is a mystery with no obvious solutions. Arif Babul of the University of Victoria said that "Observations like those of Abell 520 are humbling in the sense that in spite of all the leaps and bounds in our understanding, every now and then, we are stopped cold." UC Davis astronomer James Jee, who led the new Hubble Study, added:

"This result is a puzzle. Dark matter is not behaving as predicted, and it's not obviously clear what is going on. Theories of galaxy formation and dark matter must explain what we are seeing. We know of maybe six examples of high-speed galaxy cluster collisions where the dark matter has been mapped. But the Bullet Cluster and Abell 520 are the two that show the clearest evidence of recent mergers, and they are inconsistent with each other. No single theory explains the different behavior of dark matter in those two collisions. We need more examples."

There are about six or so explanations the astronomers have put forward to account for what's going on in Abell 520, but all either present major complications to current theory or rely on some fairly unlikely circumstances to work. One idea that preserves the current model is to suggest that Abell 520 is the result of three galaxy clusters merging together as opposed to two, explaining why it looks different from the Bullet Cluster and other such mergers. Of course, invoking a triple merger isn't a terribly elegant solution. The most audacious explanation is that some dark matter is "sticky." As the Hubble site explains:

Like two snowballs smashing together, normal matter slams into each other during a collision and slows down. But dark matter blobs are thought to pass through each other during an encounter without slowing down. This scenario proposes that some dark matter interacts with itself and stays behind when galaxy clusters collide.

That one would have some pretty massive implications if it's actually true. A more mundane possibility is that the dark core actually contains lots of galaxies, but for whatever reason they are so dim that they escaped detection by Hubble. But for that to work, these galaxies must have formed very few stars relative to other galaxies, which again raises its own set of questions.

The good news is that the Hubble survey has yielded a ton of data for the astronomers to work with. The next step is to build a computer model that can simulate the various proposed explanations and see which lines up the best with what we have actually observed. Our current understanding of dark matter has definitely taken a bit of a beating from these new findings - but it's far from defeated.